Liquid pumps in use with analytical measuring instruments require a high flow accuracy. In order to meet this requirement the pumps must be equipped with precision valves. Usually ball and seat type valves are used. Passive valves, wherein the valve ball is lifted by the flow of liquid, are known, for example, from U.S. Pat. No. 3,810,716 or from U.S. Pat. No. 4,139,469. The valve balls are often made of ruby and the seat of sapphire. Valves, wherein the ball and seat are composed of aluminum oxide ceramics (Al.sub.2 O.sub.3), are also known. Such ceramic valves are described in U.S. Pat. No. 4,832,075 or in U.S. Pat. No. 4,862,907 or in U.S. Pat. No. 5,002,662. These ceramic valves produced by the die-pressing method have economic and other advantages over ruby/sapphire valves.
Both the ruby/sapphire and the Al.sub.2 O.sub.3 -ceramic/Al.sub.2 O.sub.3 -ceramic combinations have considerable advantages in use with a pump for high-pressure liquid chromatography. These advantages are: resistance to wear, chemical resistance to commonly used solvents, accuracy of form for good sealing properties, a high machining quality at economically efficient manufacturing costs. In high-pressure liquid chromatography it is possible, especially where ruby/sapphire valves are being used, that under the influence of acetonitrile and water a coating of organic substances forms on the valve material. As a result of this coating the ball will adhere to the seat and the surface of the seat will become contaminated. This problem can be reduced by using Al.sub.2 O.sub.3 ceramics instead of ruby or sapphire.
Besides the mentioned passive valves, active valves may also be used in pumps for analytical measuring instruments, wherein the valve ball can be activated by a valve actuator. Such an active valve is known from EP-A-0 328 696. Active valves have the advantage that no flow reversal is required to close the valve. Further, movement of the valve elements is largely independent of external influences. Either ceramics or ruby/sapphire can be used as materials for the ball and seat.
The disadvantage of the valves as described above is their great tendency to become dirty. In particular in high-pressure liquid chromatography, it is impossible to avoid that smallest particles of dirt are deposited. These particles either originate from supply vessels, become detached from the feed system or are solids entrained in the solvent. These particles of dirt can build up on the valve. Once sufficient dirt has accumulated on the sealing surface the valve ceases to be tight and must be replaced.